Powder bed materials

ABSTRACT

A powder bed material can include from 80 wt % to 100 wt % metal particles having a D50 particle size distribution value from 4 μm to 150 μm. From 10 wt % to 100 wt % of the metal particles can be surface-activated metal particles having in intact inner volume and an outer volume with structural defects. The structural defects can exhibit an average surface grain density of 50,000 to 5,000,000 per mm2.

BACKGROUND

Three-dimensional printing, sometimes referred to as 3D printing, can beused for rapid prototyping and/or additive manufacturing (AM), and caninvolve computer-controlled processes by which a printer transformsmaterials into a three-dimensional physical object. Methods of 3Dprinting have continued to develop over the last few decades andinclude, but are not limited to selective laser sintering, selectivelaser melting, electron beam melting, stereolithography, fused depositmodeling, as well as others. The demand for new techniques and materialsfor 3D printing continues to increase as applicable areas of uselikewise continue to expand and evolve.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 schematically depicts an example cross-sectional view of metalparticles before and after structural defects are introduced to an outervolume thereof, e.g., surface activation, as well as a connection bridgethat can be formed between two adjacent metal particles below themelting temperature of the metal particles in accordance with thepresent disclosure;

FIG. 2 schematically depicts an example effect of flash heating on smallmetal particles when exposed to pulsed light energy followed by rapidcooling in accordance with the present disclosure;

FIG. 3 schematically depicts an example isometric view of a system forthree-dimensional printing in accordance with the present disclosure;

FIG. 4 schematically depicts an example cross-sectional view of analternative system for three-dimensional printing in accordance with thepresent disclosure;

FIG. 5 is a flow diagram of an example method of three-dimensionalprinting in accordance with the present disclosure;

FIGS. 6A through 6D provide a series of example Scanning ElectronMicroscope (SEM) images comparing surface-activated metal particles anduntreated metal particles relative to the effectiveness of the formationof connection bridges between adjacent metal particles with applicationof heat below the melting temperature of the metal particle material inaccordance with the present disclosure;

FIG. 7 is an example graph comparing the number of connective bridgesformed with surface-treated metal particles compared to metal particlesthat remained untreated when heated at the same temperature (below themelting temperature of the metal particle material) for the sameduration of time; and

FIG. 8 is an example SEM image of a metal particle that has been exposedto flash heating using pulsed light energy in accordance with thepresent disclosure.

DETAILED DESCRIPTION

In accordance with the present disclosure, three-dimensional printing ofmetal articles can involve heating metal powder in order to sinter ormelt metal particles to form a fused article. In further detail,three-dimensional printing can be carried out using metal particles of apowder bed material and selectively printing or ejecting a binder fluidonto portions of the powder bed material in a layer by layer manner,e.g., spreading powder bed material followed by applying binder fluidand repeating, to form a green part. The green part or object can thenbe sintered or annealed to form the final metal part, e.g., the greenobject moved to an oven (or remains in place) to be heat fused.Particularly in examples where the binder fluid carries a binderpolymer, such as latex particles or some other type of polymer orpolymerizable material, the polymer can burn off, e.g., burnout, at arelatively low temperature during the sintering or annealing process.Thus, if metal particle sintering does not begin to occur near thisrelatively low temperature where the binder is burned off or becomesotherwise ineffective or decomposed, there may be a temperature gap (andthus a time gap) between the time at which the polymer becomesineffective and sintering of the metal particles together begins. Thiscan lead to the collapse or partial collapse of parts, particularly inthe case of metal particles with particularly high sinteringtemperatures, e.g., Fe, Ni, Cu, Ti alloys, etc., which sinter attemperatures above 1000° C., for example. Furthermore, even when usingmetal nanoparticle, metal salt, or metal oxide nanoparticle bindersrather than polymer binders that may be devoid of polymer or includeonly small amounts of polymer, there can be a temperature range wherebinding strength may be insufficient to maintain a desired shape ofprinted object due to insufficient density of binding contact betweenthe powder particles or excessive thermal stresses associated with thepresence of binding material.

In the present disclosure, powder bed material that can be used caninclude metal particles that are surface activated by the introductionof structural defects to an outer volume thereof. This “activation” canhave the effect of allowing sintering and/or the formation of connectionbridges between adjacent or touching metal particles at temperaturesbelow the melting temperature of the metal particle material. Forexample, without being limiting, stainless-steel can be sintered andadjacent metal particles fused together to form connection bridges at atemperature of about 1000° C. for 30 minutes, even though the meltingtemperature for stainless-steel is about 1550° C. Thus, surfaceactivation of a metal particle can often generate a lower heat fusingtemperature (or melting temperature) at the surface of the metalparticles compared to the same metal particles that have not beensurface-treated. This can reduce or eliminate the temperature gapbetween when the polymer binder becomes ineffective and the metalparticles begin to sinter together. Likewise, if using a thermallysensitive metal nanoparticle, metal salt, or metal oxide binder/reducingagent systems as the binder fluid rather than a polymer binder, surfaceactivation can also improve binding strength in some instances, asbinding between metal particles may be more likely to occur due to theeasier path for atomic diffusion that may be present at the respectivesurfaces.

In accordance with this, the present disclosure is drawn to a powder bedmaterial, including 80 wt % to 100 wt % metal particles having a D50particle size distribution value from 4 μm to 150 μm. From 10 wt % to100 wt % of the metal particles can be surface-activated metal particleshaving an intact inner volume and an outer volume with structuraldefects. The structural defects can exhibit an average surface graindensity of 50,000 to 5,000,000 per mm². In further detail, the metalparticles can be elemental metals or alloys of aluminum, titanium,copper, cobalt, chromium, nickel, vanadium, tungsten, tungsten carbide,tantalum, molybdenum, magnesium, gold, silver, iron, stainless-steel,steel, or an admixture thereof. The structural defects can be introducedby ball milling, also referred to herein as “bead milling” or simply“milling.” For example, the structural defects can be introduced bymilling with 50 μm to 500 μm milling beads harder than the metalparticles suspended in an aliphatic oil at a milling speed of 400 RPM to1000 RPM for 10 minutes to 24 hours. For example, structural defectsintroduced by milling can exhibit an average surface grain density of80,000 mm² to 2,000,000 per mm². An alternative method for introducingthe surface defects is by flash heating with a high intensity photonsource, such as a xenon flash lamp or pulsed laser. In an example, thestructural defects can alternatively be introduced by flash heating themetal particles with from 1 to 10 pulses of light energy from a xenonlamp at from 15 J/cm² to 50 J/cm². For example, structural defectsintroduced by flash heating can exhibit an average surface grain densityof 60,000 mm² to 120,000 per mm², among other levels of structuraldefects outside of this more specific range.

In another example, a material set can include a powder bed material,including 80 wt % to 100 wt % metal particles having a D50 particle sizedistribution value from 4 μm to 150 μm. From 10 wt % to 100 wt % of themetal particles can be surface-activated metal particles having anintact inner volume and an outer volume with structural defects. Thestructural defects can exhibit an average surface grain density of50,000 to 5,000,000 per mm². The material set can also include a binderfluid to provide particle adhesion to a first portion of the powder bedmaterial relative to a second portion of the powder bed material not incontact with the fluid. In one example, the binder fluid can include apolymer binder or a polymerizable binder. In another example, the binderfluid can be stable at room temperature, and can include water,dispersed metal oxide nanoparticles, and a reducing agent to reduce thedispersed metal oxide nanoparticles when heat is applied to the binderfluid. In another example, the binder fluid can include metalnanoparticles or a metal salt. The metal particles can be elementalmetals or alloys of aluminum, titanium, copper, cobalt, chromium,nickel, vanadium, tungsten, tungsten carbide, tantalum, molybdenum,magnesium, gold, silver, iron, stainless-steel, steel, or an admixturethereof. The structural defects can be introduced by ball milling with50 μm to 500 μm milling beads harder than the metal particles whilesuspended in an aliphatic oil at a milling speed of 400 RPM to 1000 RPMfor 10 minutes to 24 hours. In another example, the structural defectscan be introduced by flash heating the metal particles with from 1 to 10pulses of light energy at from 15 J/cm² to 50 J/cm². In one example, theaverage surface grain density can be from 80,000 mm² to 2,000,000 permm².

In another example, a method of three-dimensional printing can includespreading a powder bed material to form a powder layer having athickness of from 20 μm to 400 μm, wherein the powder bed materialincludes 80 wt % to 100 wt % metal particles having a D50 particle sizedistribution value from 4 μm to 150 μm. From 10 wt % to 100 wt % of themetal particles can be surface-activated metal particles having anintact inner volume and an outer volume with structural defects. Thestructural defects can exhibit an average surface grain density of50,000 to 5,000,000 per mm². The method can also include selectivelybinding a first portion of the powder bed material to form a green layerwithin the powder, and can further include building up additional greenlayers by sequentially repeating the spreading and the selectivelybinding of the powder bed material until a green three-dimensionalobject is formed. In one example, the method can include heat fusing thegreen three-dimensional object to sinter or anneal the metal particlestogether. In another example, the heat fusing can start at a temperaturefrom 0.6 to 0.8 of the melting temperature of the metal particles.

It is noted that when discussing the powder bed materials, the materialsets, or the methods of three-dimensional printing, each of thesediscussions can be considered applicable to other examples whether ornot they are explicitly discussed in the context of that example. Thus,for example, in discussing a metal particle related to the materialsets, such disclosure is also relevant to and directly supported incontext of the methods or the powder bed material, and vice versa.

In certain examples of the present disclosure, not all of the metalparticles of the powder bed material are surface-modified metalparticles with an increased number of structural defects, includingmeasurable surface grains. These surface-activated metal particles canbe admixed with virgin metal particles. Thus, though 80 wt % to 100 wt %of the powder bed material can be metal particles, from 10 wt % to 100wt % (or all) of the metal particles of the powder bed material aresurface-modified. In other examples, however, all of the metal particlespresent in the powder bed material can be surface-activated metalparticles. By treating some or all of the metal particles to activatethe surface thereof as described herein, part browning and/or connectionbridges between particles can occur at lower temperatures, amelioratingpart failures that may otherwise occur with some metals, e.g., sintercan occur well below the sintering temperature of the untreatedmaterial.

When referring to the outer volume of the metal particles, the terms“activate,” “activation,” “activated,” “surface-activated,” or the likerefers to metal particles that have been treated to introduce any of anumber of large spectrum of structural defects, such as dislocations,inclusions, voids, precipitations, point defect (interstitials andvacancies), etc.

“Surface grain density” or “average surface grain density” refers to oneway of verifying the presence of structural defects that may be presentin a metal particle. Many virgin metal particles exhibit an averagesurface grain density of up to about 20,000 per mm². In accordance withthe present disclosure, the average surface grain density of the metalparticles can be from 50,000 per mm² to 5,000,000 per mm², from 60,000per mm² to 2,000,000 per mm², from 60,000 per mm² to 120,000 per mm²,from 80,0000 per mm² to 2,000,000 per mm², from 80,000 per mm² to100,000 per mm² to 2,000,000 per mm², etc. The average surface graindensity can be measured and calculated by counting the number of grainsvisible on the outermost surface of the metal particles, averaged over anumber of random metal particles sufficient to arrive at a reliablevalue. The larger the sample, typically, the more accurate the average.In one example, 10 randomly selected metal particles can be used, ormore accurately, 100 randomly selected metal particles, or even moremetal particles can be used. In accordance with the present disclosure,metal particle surface activation can be confirmed by measuring andcalculating average surface grain density, even though the other typesof structural defects may also be present. With metal particle sampleswith both virgin metal particles and surface-activated metal particles,the average surface grain density for “activated” particles can becounted based on individual grains where grains with 20,000 grains orless are considered virgin metal particles, and metal particles withgreater than 20,000 grains can be counted as surface-modified metalparticles for purposes of averaging relative amounts (of the two typesof metal particles) and the average surface grain density within eachtype of metal particle, e.g., virgin metal particles vs.surface-activated metal particles. As a further note, crystalline grainson the surface of surface-activated particles can be observed, counted,and measured. They are clearly visible in the case of flash heating. Inthe case of ball milling, their size can be deduced from the observationof the sheared particle's surface (elongated sheared planes), and fromthe observation of grains in sintered particles.

As mentioned, the surface-modified metal particles can include an innervolume and an outer volume. The inner volume can remain intact and theouter volume can be surface modified. In one example, the maximum outervolume can have a depth (measured from a surface of the metal particles)of up to 5 μm for particles that are 20 μm or larger in diameter (orlongest length for asymmetric particles). For particles less than 20 μmor larger, the depth of the outer volume (from the surface) can bereduced based on the diameter or size measurement of the metal particle.In one example, the outer volume depth may be no more than 50% comparedto a total diameter or longest size measurement of the metal particle.For example, a 10 μm particle may have a 2.5 μm outer volume depth, with2.5 μm outer volume depth on opposing sides of the metal particle and 5μm diameter or length defining an inner volume therebetween.

Turning now to example details related to the powder bed material (whichcan be included as part of the material set), the powder bed materialcan include from 80 wt % to 100 wt % metal particles, from 90 wt % to100 wt % metal particles, from 99 wt % to 100 wt % metal particles, orcan be composed of all metal particles. If the powder bed material isnot 100 wt % metal particles, then other material that may be presentcan include other metal particles, some virgin metal particles, metalparticles of another type of metal, smaller metal particles, salts,filler material, or the like. The metal particles in the presentdisclosure can be of an elemental or alloy metal material with anactivated surface, e.g., structural defects introduced to an outervolume thereof. The structural defects can provide more effectiveparticle fusing at lower temperatures compared to particles withoutsurface activation.

As mentioned, the metal particles can be, for example, aluminum,titanium, copper, cobalt, chromium, nickel, vanadium, tungsten, tungstencarbide, tantalum, molybdenum, magnesium, gold, silver, iron,stainless-steel, steel, alloys thereof, or admixtures thereof. In manyexamples, the metal particles can include a transition metal, but thereare examples where the metal particle does not include a transitionmetal, such as in the case of aluminum. In other examples, the metalparticles can be an alloy of multiple metals or can include ametalloid(s). To illustrate, the alloy may be steel or stainless-steel.Even though steel includes carbon, it is still considered to be metalalloy in accordance with examples of the present disclosure because ofits metal-like properties and the presence of a significant portion ofelemental metal. Other metal alloys that may include some carbon orsmall amounts of non-metal dopant, metalloid, impurities, etc., can alsobe considered to be a “metal” in accordance with the present disclosureas well. Examples of elements that can be included in metal alloys orblends include H, C, N, O, F, P, S, CI, Se, Br, I, At, noble gases (He,Ne, Ar, Kr, Xe, and/or Rn), etc. Metalloids that can be included in someexamples include B, Si, Ge, As, Sb, etc. Examples of metal alloys thatcan be used as the metal particles, including their respective meltingtemperatures, include ferro-aluminum (1225-1275° C.), ferro-boron(1450-1550° C.), ferro-chromium (1350-1675° C.), ferro-manganese(1060-1225° C.), ferro-molybdenum/molybdic oxide (1665-1715° C.),ferro-niobium (1500-1550° C.), ferro-phosphorus (1250-1350° C.),ferro-silicon (1225-1325° C.), ferro-silicon-manganese (1130-1230° C.),ferro-silicon-magnesium (1210-1250° C.), ferro-silicon-zirconium(1250-1340° C.), ferrous sulfide (1150-1200° C.), ferro-titanium(1070-1480° C.), ferro-vanadium (1695-1770° C.), and ferro-tungsten(1650-2100° C.). The melting temperature ranges are exemplary and can beadjusted in some instances within these ranges based on relative metalweight ratios, grade of material, etc. With these examples provided,however, a “metal” can be an elemental metal or alloy that exhibitsproperties generally associated with metals in metallurgy, e.g.,malleability, ductility, fusibility, mechanical strength, high meltingtemperature, high density, high heat and electrical conduction,sinterable, etc.

These metal particles can exhibit good flowability within the powder bedmaterial. The shape type of the metal particles can be spherical,irregular spherical, rounded, semi-rounded, discoidal, angular,subangular, cubic, cylindrical, or any combination thereof, to name afew. In one example, the metal particles can include sphericalparticles, irregular spherical particles, rounded particles, or otherparticle shapes that have an aspect ratio from 1.5:1 to 1:1, from 1.2:1,or about 1:1. In some examples, the shape of the metal particles can beuniform or substantially uniform, which can allow for relatively uniformmelting or sintering of the particulates after the three-dimensionalgreen part or object is formed and then heat fused in a sintering orannealing oven, for example. That being stated, due to the processesused to activate or introduce defects to a surface of the metalparticles, the shapes of the metal particles can be irregular, even ifthe aspect ratio may remain near 1:1, for example.

The particle size distribution can also vary. As used herein, particlesize refers to the value of the diameter of spherical particles, or inparticles that are not spherical, can refer to the longest dimension ofthat particle. The particle size can be presented as a Gaussiandistribution or a Gaussian-like distribution (or normal or normal-likedistribution). Gaussian-like distributions are distribution curves thatmay appear essentially Gaussian in their distribution curve shape, butwhich can be slightly skewed in one direction or the other (toward thesmaller end or toward the larger end of the particle size distributionrange). That being stated, an exemplary Gaussian distribution of themetal particles can be characterized generally using “D10,” “D50,” and“D90” particle size distribution values, where D10 refers to theparticle size at the 10th percentile, D50 refers to the particle size atthe 50th percentile, and D90 refers to the particle size at the 90thpercentile size. For example, a D50 value of 25 μm means that 50% of theparticles (by weight percent) have a particle size greater than 25 μmand 50% of the particles have a particle size less than 25 μm. A D10value of 10 μm means that 10% of the particles are smaller than 10 μmand 90% are larger than 10 μm. A D90 value of 50 μm means that 90% ofthe particles are smaller than 50 μm and 10% are larger than 50 μm.Particle size distribution values are not necessarily related toGaussian distribution curves, but in one example of the presentdisclosure, the metal particles can have a Gaussian distribution, ormore typically a Gaussian-like distribution with offset peaks at aboutD50. In practice, true Gaussian distributions are not typically present,as some skewing can be present, but still, the Gaussian-likedistribution can still be considered to be essentially referred to as“Gaussian” as used conventionally.

In accordance with this, in one example, the metal particles can have aD50 particle size distribution value ranging from 4 μm to 150 μm, 20 μmto 150 μm, from 20 μm to 100 μm, or from 30 μm to 80 μm, for example. Inanother example, the metal particles can have a D10 particle sizedistribution from 5 μm to 50 μm, or from 10 μm to 30 μm. In stillfurther detail, the metal particles can have a D90 particle sizedistribution from 25 μm to 85 μm, or from 35 μm to 75 μm, for example.

The metal particles can be produced using any manufacturing method.However, in one example, the metal particles can be manufactured by agas atomization process. During gas atomization, a molten metal isatomized by inert gas jets into fine metal droplets that cool whilefalling in an atomizing tower. Gas atomization can allow for theformation of mostly spherical particles. In another example, the metalparticles can be manufactured by a liquid atomization process.

By way of example, FIG. 1 depicts various metal particles 10 inaccordance with the present disclosure that can be surface-treated toform surface-activated metal particles 20 with structural defects 28. Infurther detail, two metal particles that are in close proximity, e.g.,touching, can be heated to form a physical connection bridge 30 at atemperature often well below the melting temperature of the metalparticles. The surface-activated metal particles can include an innerportion 22 that remains free of surface-introduced defects and an outerportion 26 that includes the structural defects 28, such as at a depthof up to about 5 μm.

In some examples, the powder bed material shown in FIG. 1 can be part ofa material set and/or used in a method of printing three-dimensionalobjects. In either case, the material set and/or the method can utilizea binder fluid to bind the surface-activated metal particles together ona layer by layer basis. There are various ways that can be implementedto provide metal particles with increased number of grains in the outervolume of the metal particle. As a note, increased grain density canlikewise be associated with other structural defects, such asdislocations, inclusions, voids, precipitations, point defects includingvacancies and interstitials. Average surface grain density can be viewedas a measurable proxy for many of these defects that can contribute toconnection bridge formation enhancement. With this in mind, it is notedthat surface activation can be introduced to the metal particles bymilling, flash heating, particle bombardment, e.g., neutron or protonirradiation, plasma treatment at the surface, etc. These approaches candisrupt or introduce defects to the surface of the metal particles.Rapid cooling (when approaches that add heat are used) can be used withvarious inert gases to quench the surface and enhance the surfaceactivation.

With respect to milling processes, the milling can occur in a mannerthat is vigorous enough to provide the surface density of defectsdescribed herein without adversely impacting the integrity of the innerportion of the metal particles. For example, milling may occur using 50μm to 500 μm milling beads harder than the metal particles that arebeing processed. The milling beads and metal particles can be suspendedin an aliphatic oil, e.g., Isopar® fluids from Exxon, USA, mineral oil,paraffin oil, decane, undecane, dodecane, tridecane, etc. Other oilssuch as silicon oil can also be used. A milling speed of 400 RPM to 1000RPM can be used fora relatively short period of time, e.g., 10 minutesto 24 hours. Other milling protocols can be used, provided theyintroduce structural defects to an outer volume of the metal particlesas prescribed herein without damaging an inner portion of the metalparticles so that they retain some physical integrity. Other millingbead (or ball) sizes that can be used include from 100 μm to 400 μm, orfrom 150 μm to 300 μm, for example. Other time frames that can be usedcan be from 10 minutes to 5 hours, from 20 minutes to 5 hours, from 1hour to 5 hours, from 10 minutes to 3 hours, from 20 minutes to 2 hours,etc. Other RPM speeds that can be used include from 400 RPM to 800 RPM,from 500 RPM to 1000 RPM, from 500 RPM to 800 RPM, etc. Weight ratios ofbead to metal particles can be varied based on the specific metals andbeads selected for use.

High energy ball milling can be carried out that is too vigorous toprepare the metal particles described herein. For example, ball millingthat results in the formation of nanocrystalline material goes beyondthe surface treatment contemplated in accordance with the presentdisclosure. When virgin metal particles, such as stainless-steel or someof the other metal powders described herein, are subjected to highenergy ball milling fora relatively short duration of time e.g., 10minutes to 24 hours or from 1 h to 5 h (depending on the hardness of themetal being milled), the resulting metal particles formed can exhibitintroduced surface plastic deformations, or deformations that are notelastic and do not typically return to their original shape. Severeplastic deformation, as can occur with this process, can lead to latticestrain associated with a high surface dislocation density andcorresponding increase of the surface grains without an effect (orsignificant effect) on the integrity of an inner portion or core of themetal particles. Thus, the defective lattice structure at the surfacecan possess a high amount of stored energy, which can provide an easierpath for atomic diffusion, as exemplified hereinafter by example and asshown in FIGS. 6A through 6D. As mentioned, ball milling of metalparticles is traditionally carried out to produce fully nanocrystallinepowders. In accordance with the present disclosure, the milling maystill be high energy milling, but one or more of the milling parameterscan be modified (for example, shortened milling time, reduced RPM, lowermilling media mass, etc.) to activate the surface with an inner portionor core that remains intact for providing structural integrity when themetal particles are used, e.g., for three-dimensional printing.

In other examples, flash heating (or application of pulse energy) can beused to introduce the structural defects to the outer volume of themetal particles, generating surface-activated metal particles. Oneexample of this process is shown at 40 schematically in FIG. 2 , wherevirgin metal particles 10 are exposed to pulsed energy from a flashheating source 50. This can generate a momentary molten surface that canbe rapidly gas quenched 60, e.g., helium, argon, nitrogen, neon, etc.,to leave a highly defective surface (i.e., a surface with structuraldefects 28) at an outer portion 26 of the metal particle 10 relative toan inner portion 22 thereof. The pulse energy that may be used may be aslittle as a single pulse, or can be from 1 to 10 pulses, or from 2 to 10pulses, for example. The rapid cooling can generate more grains or ahigher average surface grain density within the solidifying region thanmay occur in the case when metal particles with molten surface volumesmay be cooled more slowly. In one example, rapid cooling can be from 100microseconds to 5000 microseconds, from 500 microseconds to 2000microseconds, from 600 microseconds to 800 microseconds, etc. Thepresence of the inert gas during the cooling/quenching process canprevent the metal particles from oxidizing, in some examples. The metalparticles can be treated by placing them on a ceramic surface andirradiating them with the pulse energy, such as from a xenon lamp. Onthe other hand, and as shown by example in FIG. 2 , the irradiationprocess can include the use of multiple xenon lamps (two or more) tofocus on an irradiation region where the metal particles are droppedtherethrough under gravity, for example. When the particles are flashedwith a high energy lamp while falling through the irradiation region,the flashed energy can instantaneously (or very rapidly) heat theexposed surface (including a shallow depth or volume thereof) up to orabove its melting temperature. The high-temperature gradient at thesurface of the particle can cause rapid quenching and formation of alarge number of small grains which do not have enough time to coalesceinto a smaller number of larger grains. Rapid quenching can also lead tothe formation of a partially amorphous metal particle structure. Therecan be other conceivable methodologies that could likewise be used.

Regardless of the method used to introduce the structural defects, insome specific examples, the structural defect depth of the metalparticles can be up to about 5 μm, up to about 4 μm, up to about 3 μm,up to about 2 μm, up to about 1 μm, or up to about 0.5 μm. In someexamples, the defect depths can be limited by the size of the metalparticle being used. For example, the structural defect depth can belimited so that at least half of the particle size can be retainedwithout the structural defects. To illustrate, a 4 μm metal particle canhave a structural defect depth up to 1 μm, leaving a 2 μm inner portionintact, a 10 μm metal particle can have a structural defect depth up to2.5 μm leaving a 5 μm inner portion intact, a 20 μm metal particle canhave a structural defect depth up to 5 μm leaving a 10 μm inner portionintact, etc. As the defect depth may also have a total depth of up toabout 5 μm, any particle size over about 20 μm can have any structuraldefect depth up to about 5 μm, for example. These ranges are provided byexample, as in some instances, structural defects depths outside ofthese ranges can be prepared.

FIG. 3 depicts a three-dimensional printing system 100 that uses apowder bed material 106 including from 10 wt % to 100 wt % of thesurface-activated metal particles as described herein, and a binderfluid 102 which is applied to the powder bed material on a layer bylayer basis. More specifically, the powder bed material can be used toprepare three-dimensional green parts. To print a part, the powder bedmaterial is layered as mentioned. During the printing of the (green)part, a new top layer 116 of powder bed material is applied to anexisting substrate (either the build platform that supports the powderbed material 106, or previously deposited powder bed material 106, orpreviously generated green layer), and in this example, is flattenedusing a roller 104. The binder fluid 102, which is contained and ejectedfrom a fluid ejector 110, such as a digital jetting pen, e.g. thermalfluid ejector, can then be applied to the top layer of the powder bedmaterial 106 in a pattern 114 which corresponds to a layer of thethree-dimensional object that is being built. In some examples, whereapplicable, the top layer of powder bed material 106 with the binderfluid 102 printed thereon (or within some or all of the top layer) canthen be exposed to energy from an energy source 112 to cause the binderfluid 102 to bind the powder bed material 106 together at the pattern(and not outside of the printed pattern). In one example, the energy canbe IR or UV energy suitable to initiate binder polymerization, flashheating energy from a pulsed light source, e.g., a xenon lamp, etc. As anote, in some examples, additional energy may be added, or in otherexamples, may not be added above the thermal energy that may already bepresent during printing, e.g., up to 200° C. The process can then berepeated to generate a three-dimensional green part or object that canbe later heat fused in an oven or by some other heating technique.

FIG. 4 illustrates schematically a related three-dimensional printingsystem 200 in accordance with examples of the present disclosure. InFIG. 4 , the system can include a powder bed material 206 (of thesurface-activated metal particles with structural defects introduced bymilling, flash heating, etc.), a build platform 208, a fluid ejector210, an energy source 212 for generating and applying energy to thepowder bed material, e.g., after application of a binder fluid from thefluid ejector, and a powder material source 218 for supplying a newlayer 216 of powder bed material for facilitating the build. In thisexample, the build platform 208 acts as the substrate for the firstlayer applied, and layers of powder bed material 206 and green part orobject layer act as the substrate for subsequently applied powder bedmaterial layers. Thus, the term “build platform” can refer to a rigidsubstrate that is used to support powder bed material 206 during thethree-dimensional printing process. The build platform 208 can have sidewalls, for example, to retain the powder bed material 206, in oneexample. The more generic term “substrate,” on the other hand, can referto a build platform, powder bed material that may have already beendeposited to the build platform, or any previously deposited powder bedmaterial that has been bound together by the binder fluid to form agreen layer of the green part or object that is being formed. In thisexample, for reference, a printed article 214 is also shown that can beprinted using the present layer by layer printing process. As shown, thepowder bed material 206 (either bound together using the binder fluid oras unprinted free flowing powder bed material) can sequentially supportnew layers during the build process. The powder bed material 206 can bespread as a 25 μm to 400 μm layer of the powder bed material 206 in thepowder bed. Then the fluid ejector 210 can eject a fluid over selectedsurface regions of the powder bed material 206, and then, in someinstances, additional energy can be applied to heat or initiate areaction at the powder bed material.

In further detail, regarding the binder fluid that may be present in thematerial set or printing methods described herein, any of a number ofbinders carried by a liquid vehicle for dispensing on the powder bedmaterial can be used. The term “binder” includes material used tophysically bind separate metal particles together or facilitate adhesionto a surface of adjacent metal particles to a green part or object inpreparation for subsequent sintering or annealing. The binder fluid canprovide binding to the powder bed material upon application, or in someinstances, can be further treated after printing to provide bindingproperties, e.g., exposure to IR energy to evaporate volatile species,exposure to flash heating (photo energy and heat) to activate a reducingagent, exposure to UV or IR energy to initiate polymerization, etc. A“green” part or object (or individual layer) refers to any component ormixture of components that is not yet sintered or annealed. Once thegreen part or object is sintered or annealed, the part or object can bereferred to as a “brown” object or part. “Sintering” refers to theconsolidation and physical bonding of the metallic particles together(after temporary binding using the binder fluid) by solid statediffusion bonding, partial melting of one or more phases or metalparticles present, or a combination of solid state diffusion bonding andpartial melting. The term “anneal” refers to a heating and coolingsequence that controls not only the heating process, but the coolingprocess, e.g., slowing cooling in some instances, to remove internalstresses and/or toughen the sintered part or object (or “brown” part)prepared in accordance with examples of the present disclosure.Furthermore, for some surface-activated metal particles, the structuraldefects can be introduced so that the presence of polymeric binder canbe eliminated altogether. In other words, the binder fluid in someexamples can be free of polymeric binder.

With more specific reference to the various types of binder fluid thatcan be used, in one example, the binder fluid can include a polymericbinder that provides the binding properties when ejected or printed ontothe powder bed material. The polymer can be, for example, a latexpolymer that is fluid-jettable from a fluid ejector, such as a piezo orthermal inkjet pen. Example latex polymer particle size can be from 10nm to 200 nm, and the concentration of the latex particles in the binderfluid can be from 0.5 wt % to 20 wt %, for example. Other binder fluidsmay include pre-polymer material that may be polymerized after ejectiononto the powder bed material. In one example, the binder fluid mayinclude water soluble acrylate- or methacrylate-based monomer carried byan aqueous liquid vehicle. For example, a binder fluid may include amonofunctional acrylate- or methacrylate-based monomer, a water solubledifunctional acrylate- or methacrylate-based monomer, an amine, andwater. For example, the monofunctional monomer can be 2-hydroxy ethylmethacrylate (IEEMA) or other similar monomer, and the difunctionalmonomer can be glycerol dimethacrylate or other similar monomer. Anexample amine that can be used is N,N-dimethyl-4-ethyl benzoate or othersimilar amine compound. In some examples, an initiator can be present,such as a photoinitiator (e.g. UV or IR) for initiating the reaction ofthe various monomers and amines, etc., during layer by layer binderfluid deposition. Other polymers that may be suitable for use in thebinder fluid can include poly(meth)acrylates, polyvinyl alcohols,polyvinyl acetates, polyvinyl pyrrolidones, polyvinyl butyrals, etc.Organo-metallic polymers, polysilanes, polycarbosilanes, polysilazanes,waxes, or other similar binder material can also be formulated into thebinder fluids of the present disclosure. With polymeric binder fluids,typically, the binder contained therein can undergo a burnout processwhere the polymer essentially burns off during the sintering orannealing process. The term “burnout” refers to thermal binder burnoutwhere thermal energy to a green part or object removes inorganic ororganic volatiles and/or other materials that may be present. Burnoutmay result in some or all of the non-metal material to be removed. Insome systems, burnout may not occur, such as in the instance where thebinder is a metal oxide and the reducing agent is consumed in theredox-reaction.

In further detail, regarding the binder fluid, it is notable that therecan be multiple binder fluids used in some examples, and/or the binderfluid can include more than one binder material. With multiple binderfluids, the various types of binders carried by the multiple binderfluids, respectively, can be selected to provide multistep binding. Forexample, a first binder may melt and bind within a relative lowtemperature range, and as the temperature rises and the first binderfails, the second binder may then melt and start to contribute to thebinding within a second range of temperatures, and so forth, e.g., twobinders, three binders, four binders, etc. In the present disclosure,even if the highest melting temperature binder were to fail before thesurface-activated metal particle were to reach sintering temperatures,the structural defects (and easier path for atomic diffusion that may bepresent) can close the temperature gap between the highest bindermelting temperature and the sintering temperature where connectionbridges begin to form.

In other examples, binder fluids may be prepared that do not rely onpolymers for providing the binding properties prior to sintering. Thesesystems can include some (reduced concentration) of polymer or can bedevoid of polymer altogether. For example, the binder fluid can be athermally sensitive binder fluid that includes an aqueous liquidvehicle, a reducible metal compound, and a thermally activated reducingagent. In this example, the water can be present at from 20 wt % to 95wt %, from 30 wt % to 80 wt % water, or from 50 wt % to 80 wt %. Thereducible metal compound can be present at from 2 wt % to 40 wt %, from7 wt % to 30 wt %, or from 10 wt % to 35 wt %. The thermally activatedreducing agent can be present at from 2 wt % to 40 wt %, from 7 wt % to30 wt %, or from 10 wt % to 35 wt %.

In further detail, the reducible metal compound can be reduced byhydrogen released from the thermally activated reducing agent. Anotherpossible mechanism can include the formation of radicals that attack themetal compound (e.g., metal oxide) and reduce it to a pure metal.Decomposition of the reactive agent can be very fast or instantaneous,driven by the high energy pulse (highly thermodynamicallynon-equilibrium process) and it may produce transient moieties capableof attacking the metal compounds. Examples of reducible metal compoundscan include metal oxides (from one or more oxidation state), such as acopper oxide, e.g., copper I oxide or copper II oxide; an iron oxide,e.g., iron(II) oxide or iron(III) oxide; an aluminum oxide, a chromiumoxide, e.g., chromium(IV) oxide; titanium oxide, a silver oxide, zincoxide, etc. As a note, due to variable oxidation states of transitionmetals, they can form various oxides in different oxidation states,e.g., transition metals can form oxides of different oxidation states.

In other examples, the binder fluid can include organic or inorganicmetal salts. In particular, inorganic metal salts that can be usedinclude metal bromides, metal chlorides, metal nitrates, metal sulfates,metal nitrites, metal carbonates, or a combination thereof. Inorganicmetal salts can include chromic acid, chrome sulfate, cobalt sulfate,potassium gold cyanide, potassium silver cyanide, copper cyanide, coppernitrate, copper sulfate, iron acetate, iron nitrate, nickel carbonate,nickel chloride, nickel fluoride, nickel nitrate, nickel sulfate,potassium hexahydroxy stannate, sodium hexahydroxy stannate, silvercyanide, silver ethansulfonate, silver nitrate, sodium zincate, stannouschloride (or tin(II) chloride), stannous sulfate (or tin(II) sulfate,zinc chloride, zinc cyanide, tin methansulfonate, for example. In someinstances, the reducible metal compound can be in the form of ananoparticle, and in other instances, the reducible metal compound canbe disassociated or dissolved in the aqueous liquid vehicle, e.g.,copper nitrate or copper chloride. As nanoparticles, the reducible metalcompound can have a D50 particle size from 10 nm to 1 μm, from 15 nm to750 nm, or from 20 nm to 400 nm. In some instances, small nanoparticlescan be used, such as those from 10 nm to 200 nm. Thermally sensitivebinder fluids can be digitally ejectable from a fluid ejector withreliability, such as a piezoelectric fluid ejector or even a thermalfluid ejector in some examples.

The reducing agent can be particularly sensitive to rapidly appliedelevated temperatures and may also be activated by a photochemicalreaction introduced by flash heating. The term “flash” heating (orfusion) or application of “pulse energy” refers to raising a temperatureof a surface layer of a powder bed material using photo energy while incontact with a binder fluid printed thereon (or therein) in a durationof few (or less) milliseconds. Flash heating can be tuned, for example,to have little to no impact on the already applied underlying greenlayer or powder bed material of the printed object, except in someinstances perhaps to assist in adhering a newly formed layer to thesubsequently applied and flash heated layer. Flash heating can, in otherexamples, have some impact on lower layers, depending on the materialand the layer thickness. Example pulse energies that can be irradiatedby a flash or pulse light source, particularly when the binder includesa reducible metal compound and a reducing agent, can be from 15 J/cm² to50 J/cm² (positioned from 5 mm to 150 mm away from the powder bedmaterial), or from 20 J/cm² to 40 J/cm². For example, the light sourcecan be a non-coherent light source such as a pulsed gas discharge lamp.In further detail, the light source can be a commercially availablexenon pulse lamp. The light source can alternatively be capable ofemitting a pulse energy at an energy level(s) from 20 J/cm² to 45 J/cm².In other examples the light source can be positioned at from 25 mm to125 mm, 75 mm to 150 mm, 30 mm to 70 mm, or 10 mm to 20 mm away from thepowder bed material during operation. It should also be noted thatpulsing the light energy (or flash heating) can be based on a singlepulse or repeated pulses as may be designed for a specific applicationor material set to advance the binding properties of the printed binderfluid, e.g., initiate polymerization, initiate redox reaction. Toillustrate, a higher energy single pulse may be enough to cause afast-redox reaction to occur, or multiple lower energy pulses canlikewise be used if a slower redox reaction may be desired (per layer),e.g., from 2 to 1000 pulses, from 2 to 100 pulses, from 2 to 20 pulses,from 5 to 1000 pulses, from 5 to 100 pulses, etc.

Example thermally activated reducing agents can include hydrogen (H₂),lithium aluminum hydride, sodium borohydride, a borane (e.g., diborane,catecholborane, etc.) sodium hydrosulfite, hydrazine, a hindered amine,2-pyrrolidone, ascorbic acid, a reducing sugar (e.g., a monosaccharide),diisobutylaluminium hydride, formic acid, formaldehyde, or mixturesthereof. The choice of reducing agent can be such that it is thermallyactivated as may be dictated by the choice of the thermally reduciblemetal compound, e.g. to keep the metal oxide or salt primarily in itsnative or original state (as an oxide or salt) until their reaction withthe reducing agent is desired at the elevated temperatures describedherein, e.g., at flash heating. If the reducing agent and the metaloxide or salt is too reactive, e.g., at room temperature, the reduciblemetal compound (oxide or salt) can become reduced prematurely in thebinder fluid leaving behind reduced metal nanoparticles that couldeasily degrade by contact with air/moisture.

In this specific example, the binder fluid of this type, if used, can bereferred to as a “thermally sensitive” binder fluid, meaning the metaloxide or salt is not reduced until printed in a powder bed material andthen exposed to rapid heat increases by flash heating. That beingstated, some polymers in binder fluids can also be thermally sensitive,in that they melt above application temperature to provide bindingproperties. Thus, flash heating can be used for thermally sensitiveand/or photoreactive binder fluids that include polymeric binder aswell. If using flash heating for causing a reducing agent to react witha reducible metal compound, e.g., metal oxide, the powder bed materialhaving the binder fluid printed to a layer thereof can be exposed tohigh temperatures, such as an essentially instantaneous high reactiontemperature, e.g., from 200° C. to 1000° C., from 250° C. to 1000° C.,from 300° C. to 700° C., etc. Polymer binders can be exposed to thesetypes of temperatures as well, but in some examples, lower temperatureranges with lower limits can be used as well, e.g., from 80° C. to 600°C., from 100° C. to 500° C., from 200° C. to 400° C., etc. With thebinder fluids that include polymer binder, other methods of heating canbe used, as in some cases, the temperatures may be lower and more easilyraised to applicable softening and/or melting temperatures. Regardless,if using flash heating, raising the temperature rapidly can acceleratemelting and/or redox-reaction that may occur to cause binding of powderbed material to occur.

Flash heating (using a flash pulse power source, for example) cangenerate high temperatures with efficiency, as a flash heating processcan be tuned to facilitate heating to any temperature above roomtemperature up to even a melting temperature of many metals. That beingmentioned, reducing the reducible metal compound in the presence of athermally sensitive reducing agent can be carried out at a temperaturewell below the melting temperature of the metal, thus providing metalbinder to join or adhere powder bed metal particles together in asufficiently strong manner to allow for further processing, e.g., ovenheating, sintering, annealing, etc.

In further detail, in order to generate three-dimensional printed parts,such as green parts or finished heat fused parts, three-dimensionalpowder-bed printing can be carried out a layer at a time. To illustrate,a layer of the powder bed material can be deposited and spread outevenly on a substrate, e.g., a build platform, a previously appliedlayer of powder bed material, or a previously formed green layer,typically evenly at the top surface. The layer of powder bed materialcan be from 25 μm to 400 μm, from 75 μm to 400 μm, from 100 μm to 400μm, 150 μm to 350 μm, or from 200 μm to 350 μm, for example. Thethickness of the layer can be determined in part based on the powder bedmaterial particle size or particle size distribution, e.g., D50 particlesize, etc., and/or upon the desired resolution of the printed part,and/or the amount of binder fluid applied to (or into) an uppermostlayer of the powder bed material. Next, the binder fluid can then beselectively printed on a portion of the powder bed material in a desiredpattern corresponding to a layer of the three-dimensional part or objectto be printed. This can be carried out at a relatively low temperature(temperature typically below 200° C.). Notably, elevated temperature canprovide some removal (evaporation) of volatile liquid components of thebinder fluid, e.g., elevated above about 100° C. Next, the powder bedmaterial layer printed with binder fluid, in some instances, can beprocessed further, e.g., exposed to UV or IR energy to initiatepolymerization, flash heated by exposing to a pulse of light or opticalenergy to initiate polymerization or initiate a redox-reaction, etc.Once the three-dimensional green part or object is formed, the greenpart or object can be transferred or otherwise heated in a moretraditional oven, such as an annealing oven or a sintering oven. There,the metal particles of the powder bed material (bound together with oneor more of the various binders or binder systems, e.g., binder fluidwith energy input, etc.) can become sintered together, or otherwise forma more permanent structure or rigid metal part or object (or “brown”part) compared to the green part. In the sintering or annealing oven,volatile byproducts not already removed during printing, e.g., typicallybelow 200° C., may be further removed as the temperature increases.

FIG. 5 depicts a method of three-dimensional printing 300, which caninclude spreading 310 a powder bed material to form a powder layerhaving a thickness of from 20 μm to 400 μm. The powder bed material caninclude 80 wt % to 100 wt % metal particles having a D50 particle sizedistribution value from 4 μm to 150 μm. From 10 wt % to 100 wt % of themetal particles can be surface-activated metal particles having inintact inner volume and an outer volume with structural defectsexhibiting a surface grain density of 50,000 to 5,000,000 per mm². Themethod can further include selectively binding 320 a first portion ofthe powder bed material to form a green layer within the powder layer,and building up 330 additional green layers by sequentially repeatingthe spreading and the selectively binding of the powder bed materialuntil a green three-dimensional object is formed.

It is noted that, as used in this specification and the appended claims,the singular forms “a,” “an,” and “the” include plural referents unlessthe content clearly dictates otherwise.

As used herein, the term “about” is used to provide flexibility to anumerical range endpoint by providing that a given value may be “alittle above” or “a little below” the endpoint. The degree offlexibility of this term can be dictated by the particular variable anddetermined based on the associated description herein.

As used herein, “aspect ratio” refers to an average of the aspect ratioof the collective particles as measured on the individual particle bythe longest dimension in one direction and the longest dimension in aperpendicular direction to the measured dimension.

“Particle size” refers to the diameter of spherical particles, or to thelongest dimension of non-spherical particles. When the metal particle isnot spherical or is asymmetrical, the longest dimension can also be usedto establish the relative size of the inner volume relative to the depthof defects in the outer volume.

As used herein, “first” and “second” are not intended to denote order.These terms are utilized to distinguish an element, component, orcomposition from another element, component, or composition. Thus, theterm “second” does not infer that there is a “first” within the samecompound or composition, but rather it is merely a “second” element,compound, or composition relative to the “first.”

As used herein, a plurality of items, structural elements, compositionalelements, and/or materials may be presented in a common list forconvenience. However, these lists should be construed as though eachmember of the list is individually identified as a separate and uniquemember. Thus, no individual member of such list should be construed as ade facto equivalent of any other member of the same list solely based ontheir presentation in a common group without indications to thecontrary.

Concentrations, dimensions, amounts, and other numerical data may bepresented herein in a range format. It is to be understood that suchrange format is used merely for convenience and brevity and should beinterpreted flexibly to include not only the numerical values explicitlyrecited as the limits of the range, but also to include all theindividual numerical values or sub-ranges encompassed within that rangeas if each numerical value and sub-range is explicitly recited. Forexample, a weight ratio range of 1 wt % to 20 wt % should be interpretedto include not only the explicitly recited limits of 1 wt % and 20 wt %,but also to include individual weights such as 2 wt %, 11 wt %, 14 wt %,and sub-ranges such as 10 wt % to 20 wt %, 5 wt % to 15 wt %, etc.

EXAMPLES

The following examples illustrate several alternatives in accordancewith the present disclosure. However, it is to be understood that thefollowing is only illustrative of the application of the principles ofthe present disclosure. Numerous modifications and alternativecompositions, methods, and systems may be devised without departing fromthe spirit and scope of the present disclosure. The appended claims areintended to cover such modifications and arrangements.

Example 1—Preparing and Sintering Surface-Activated Metal Particles byMilling

Virgin stainless-steel metal particles were compared with ball-milledstainless-steel particles to determine relative sintering and heatfusing behavior relative to one another. FIG. 6A provides an example SEMimage of a virgin stainless-steel metal particle 10 before sintering theparticles at 1000° C. for 30 minutes in argon gas. FIG. 6B provides anexample SEM image of the virgin stainless-steel metal particle 10 aftersintering. Virgin stainless-steel metal particles can be identified asincluding dendrites 12, which are branch like structures often formed onthe surface stainless-steel balls that have not been surface modified.As can be seen, after sintering at this temperature (which is about 550°C. below the melting temperature of stainless-steel), the steel ballswere not able to form connection bridges between adjacent metalparticles (30 shown in FIG. 6B). FIG. 6C also provides SEM images ofball-milled stainless-steel metal particles 20 before sintering at thesame temperature and time frame. FIG. 6D provides SEM images of theball-milled stainless-steel metal particles 20 after sintering. As canbe seen in the before image (20 shown at A), the dendrites are notdiscernable on the outer portion 26 of the ball-milled metal particlesand structural defects 28 (activation) are now visible in the SEM image.A comparison of the stainless-steel balls after exposure to 1000° C. for30 minutes reveals that the metal particles that were ball-milled formedconnection bridges 30 and the virgin metal particles that were not ballmilled formed no connection bridges. Thus, during sintering, thesurface-activated stainless-steel particles seem to provide fasteratomic diffusion rates even at lower temperatures (below the meltingtemperature of stainless-steel). This can lead to the early formation ofconnection bridges or “necks” between metal particles, which can providefaster and/or earlier brown part densification.

FIG. 7 provides data comparing the number of connection bridges thatwere formed on a particle by particle basis. As can be seen in theconnection bridge graph shown in this FIG., at 1000° C. for 30 minutesin an argon gas environment, the virgin stainless-steel particles didnot form any connection bridges between adjacent particles, whereas withthe milled stainless-steel particles, a large number of connectionbridges were formed.

Example 2—Preparing Surface-Activated Metal Particles by Flash Heating

Stainless-steel metal particles are flash heated by applying the metalparticles on a ceramic plate and applying a single pulse (30 J/cm²) offlash energy, or by suspending the metal particles in a medium andapplying multiple pulses (25 to 30 J/cm²) of flash energy to theparticles. The flash energy or flash heating is applied using ahigh-intensity xenon lamp. An example of a stainless-steel metalparticle that has been irradiated by pulse energy is shown in FIG. 8 ,where a clear contrast with respect to structural defects can beobserved compared to the virgin stainless-steel shown in FIG. 4 (virginstainless-steel metal particle shown at 10, image A). Metal particleshaving a high average surface grain density and associated increasednumber of structural defects (dislocation, voids, inclusion, etc.) canbe capable of faster formation of connection bridges at lowertemperatures between the adjacent metal particles during the sinteringor annealing process.

Example 2—Binder Fluid

An acrylic latex binder fluid is prepared that includes from 10 wt % to20 wt % acrylic latex binder particles and a liquid vehicle which ispredominantly water and other volatiles. This acrylic latex binder fluidis ejectable from a thermal fluid ejector onto powder bed material.

Example 3—Three-Dimensional Printing

A powder bed material of 100 wt % milled stainless-steel particlesprepared in accordance with Example 1 are spread on a substrate and theacrylic latex binder fluid of Example 2 is printed thereon to form agreen part or object layer. Powder bed spreading and acrylic latexbinder printing is repeated until a green part or object is formed. Thegreen part or object is removed from the powder bed container andtransferred to annealing furnace. The furnace temperature was graduallyraised to accommodate first aqueous solvent evaporation (around 100° C.to 150° C.), then melting of the latex binder (around 140° C. to 250°C.). Further temperature increase from 250° C. to 400° C. (in presenceof an oxidizing ambient, then a reducing ambient) accommodates graduallatex burnout and removal of volatile byproducts that are present in theliquid vehicle. Sintering and the formation of connection bridges occurswell below the melting temperature of virgin stainless-steel. The greenpart is thus converted to a brown part well below the meltingtemperature of stainless-steel.

What is claimed is:
 1. A powder bed material, comprising: 80 wt % to 100wt % metal particles having a D50 particle size distribution value from4 μm to 150 μm, wherein 10 wt % to 90 wt % of the metal particles havean average surface grain density of 20,000 per mm² or less, and wherein10 wt % to 90 wt % of the metal particles are surface-activated to havean outer volume with increased structural defects compared to an innervolume and an average surface grain density of from 60,000 per mm² to120,000 per mm².
 2. The powder bed material of claim 1, wherein themetal particles are elemental metals or alloys of aluminum, titanium,copper, cobalt, chromium, nickel, vanadium, tungsten, tantalum,molybdenum, magnesium, gold, silver, iron, stainless-steel, steel, or anadmixture thereof.
 3. The powder bed material of claim 1, wherein thestructural defects are introduced by flash heating the metal particleswith from 1 to 10 pulses of light energy at from 15 J/cm² to 50 J/cm².4. The powder bed material of claim 1, wherein the metal particleshaving an average surface grain density of 20,000 per mm² or less aremade of the same metal as the surface-activated metal particles.
 5. Thepowder bed material of claim 1, wherein 80 wt % to 90 wt % of the metalparticles are surface activated metal particles.
 6. The powder bedmaterial of claim 1, wherein the D50 particle size distribution value ofthe metal particles ranges from 20 μm to 100 μm.
 7. The powder bedmaterial of claim 1, wherein the metal particles that are surfaceactivated are flash-heated metal particles.
 8. The powder bed materialof claim 5, wherein a balance of the metal particles has the averagesurface grain density of 20,000 per mm² or less.
 9. A material set,comprising: a powder bed material comprising 80 wt % to 100 wt % metalparticles having a D50 particle size distribution value from 4 μm to 150μm, wherein 10 wt % to 90 wt % of the metal particles have an averagesurface grain density of 20,000 per mm² or less, and wherein 10 wt % to90 wt % of the metal particles are surface-activated metal particleshaving an intact inner volume and an outer volume with increasedstructural defects compared to the inner volume and an average surfacegrain density of from 60,000 per mm² to 120,000 per mm²; and a binderfluid to adhere a first portion of the powder bed material relative to asecond portion of the powder bed material not in contact with the binderfluid.
 10. The material set of claim 9, wherein the binder fluidincludes water and a polymer binder or a polymerizable binder.
 11. Thematerial set of claim 9, wherein the binder fluid is stable at roomtemperature and includes water, dispersed metal oxide nanoparticles, anda reducing agent to reduce the dispersed metal oxide nanoparticles whenheat is applied to the binder fluid.
 12. The material set of claim 9,wherein the structural defects are introduced by flash heating the metalparticles with from 1 to 10 pulses of light energy at from 15 J/cm² to50 J/cm².
 13. A method of three-dimensional printing, comprising:spreading a powder bed material to form a powder layer having athickness of from 20 μm to 400 μm, wherein the powder bed materialincludes 80 wt % to 100 wt % metal particles having a D50 particle sizedistribution value from 4 μm to 150 μm, wherein 10 wt % to 90 wt % ofthe metal particles have an average surface grain density of 20,000 permm² or less, and wherein 10 wt % to 90 wt % of the metal particles aresurface-activated metal particles having an intact inner volume and anouter volume with increased structural defects compared to the innervolume and an average surface grain density of from 60,000 per mm² to120,000 per mm²; and selectively binding a first portion of the powderbed material to form a green layer within the powder layer; and buildingup additional green layers by sequentially repeating the spreading andthe selectively binding of the powder bed material until a greenthree-dimensional object is formed.
 14. The method of claim 13, furthercomprising heat fusing the green three-dimensional object to sinter oranneal the metal particles together.
 15. The method of claim 14, whereinthe heat fusing starts at a temperature from 0.6 to 0.8 of the meltingtemperature of the metal particles.
 16. A powder bed material,comprising: 80 wt % to 100 wt % metal particles having a D50 particlesize distribution value from 4 μm to 150 μm, wherein 80 wt % to 99 wt %of the metal particles are surface-activated to have an outer volumewith increased structural defects compared to an inner volume and asurface grain density of from 50,000 per mm² to 5,000,000 per mm², andwherein a balance of the metal particles have an average surface graindensity of 20,000 per mm² or less.
 17. The powder bed material of claim16, wherein the metal particles are elemental metals or alloys ofaluminum, titanium, copper, cobalt, chromium, nickel, vanadium,tungsten, tantalum, molybdenum, magnesium, gold, silver, iron,stainless-steel, steel, or an admixture thereof.
 18. The powder bedmaterial of claim 16, wherein the D50 particle size distribution valueof the metal particles ranges from 20 μm to 100 μm.
 19. The powder bedmaterial of claim 16, wherein the metal particles that are surfaceactivated are flash-heated metal particles or ball-milled metalparticles.